Architectures for standard cell libraries, as well as gate array libraries, have become fairly standard over the years. Most common architectures employ first level power buses that run continuously through the cell. An example of such a cell is shown in FIG. 1, where the power rails, 101 and 102 run along the top and bottom edge of the cell. The individual cells, 100 and 103, are placed adjacent to one another so that the power rails, 101 and 102, run continuously from one end of the cell row to the other. In the event that there is a gap in the cell row, the bus is made to be continuous either by use of a filler cell or by routing a wire through the gap. This power bus can be called an “explicit bus” since each cell is explicitly connected to each other cell by the actual placement of the cell itself.
The cell rows themselves are tied together in a grid using other levels, as shown in FIG. 2. The first layer busses, 110, are strapped vertically in a second level using a wider bus, 111. The second level is, in turn, strapped less frequently horizontally by a third layer using an even wider bus, 112, and so on. In this structured approach, each bus of a given layer typically has the same width, regardless of the power requirements of a given section of the grid.
In FIG. 3, a more complete definition of the common standard cell library architecture is given. Fixed width power busses, 120, run in first level along the top and bottom edge of the cell. These cells are referred to as “gridded cells” because they are built on a “construction grid”, 122, which is typically defined at the via-to-via or via-to-wire spacing for the technology. This construction grid is on the order of several times the gate length of the technology used, typically from 2 to 4 times the drawn gate length. The gridded cell must have most features, including port locations, 121, and cell boundary box, 123, lie on the fixed construction grid. In some cases, these restrictions can make a given cell larger or more difficult to connect to than a cell built without using any type of construction grid.
In most standard cell libraries, the contents of the cells must lie entirely within the cell to avoid creating illegal interactions with adjacent cells. This is illustrated in FIG. 4. Two cells, 130, are placed next to one another, 138, by placing their boundary boxes, 134 and 139, adjacent to one another. The power busses, 131 and 137, join together and become continuous. However, the source diffusion areas in the center, 133 and 135, and the source contacts, 132 and 136, cannot be shared, even though they are electrically connected to the same net.
In FIG. 5, examples of standard cells are shown, 140, which contain “tap contacts”. These are electrical connections either to the well, as in 142, or to the substrate, as in 143. These ties electrically couple the appropriate power bus, 141, to the respective well or substrate. While this is an electrical requirement, to include the tap contacts in the cells themselves can use valuable space and make the cell larger or limit the device sizes that can be drawn in the cells.